The first successful experiment to pump lasers with nuclear radiation occurred on May 12, 1972 at VNIIEF using a He-Xe mixture, but VNIIEF researchers were not able to publish their first article about NPLs until 1979 [5]. This article contained the results of later experiments, while information about first experiments was briefly presented later in a survey paper [7].

In the first series of experiments, the pulsed water reactor VIR-2 [2, 33], which was used for research from 1971 through 1978, was used as the neutron source. In 1979, the modified reactor VIR-2 M [2, 33], with a strengthened reactor core and minor changes in the parameters of the reactor pulse, was placed in operation. A solution of uranium salt (UO2SO4) in ordinary water served as the fuel in these reactors.

The arrangement of the VIR-2 and VIR-2 M reactors in the building is shown in Fig. 2.5. The reactor is located in the two-hall building having a concrete wall thickness of 2-6 m. The reactor core vessel (height 2 m; diameter roughly 0.7 m; wall thickness 65 mm) was enclosed in a concrete block measuring 4 x 4 x 3.5 m, which was the biological shielding. The bottom of the vessel is at the level of the lower hall’s ceiling, and can be closed by a protective shutter. The following experimental channels are used to locate the following exposed objects:

(a) A central channel with an internal diameter of 142 mm.

(b) A hemispherical cavity with an internal diameter of 300 mm.

(c) Side channels with a diameter of 100 mm abutting the side surface of the reactor core vessel.

(d) A cavity close to the surface of the reactor core with a cross-section of 560 x 620 mm2.

(e) A lower reactor hall with a height of 2.5 m.

Figure 2.6 shows a diagram of the first experiments. The vertically arranged laser cell was irradiated in the side channel of the VIR-2 reactor. The average fluence of the thermal neutrons along the length of the cell was 1.3 x 1013 cm~2 with a reactor pulse duration of around 4 ms.

An aluminum tube was placed inside the cylindrical body of the cell with an internal diameter of 27 mm and a length of 100 cm. A layer of 235U3O8 was deposited on the tube’s surface with a thickness equivalent to around 2-3 mg/cm2

of metallic 235U. To increase the flux of the thermal neutrons, the cell was enclosed by a polyethylene moderator with a wall thickness of 30 mm. The laser cavity was formed by mirrors with a silver coating: a spherical 100 % reflectivity mirror with a radius of curvature of 10 m and a flat output coupler on quartz or BaF2 substrate.

Fig. 2.7 Oscillogram of a reactor pulse (upper trace) and laser radiation pulse (lower trace) for He-Xe mixture at a pressure of

1 atm. Division value is

2 ms

The laser radiation is extracted from the cell through a coupling hole with a diameter of 1-2 mm in the flat mirror. The distance between the mirrors is 120 cm.

The measurement devices were kept away from the irradiation zone at a distance of around 10 m from the laser cell. In the searching experiments, a FEU-28 photomultiplier was used to register radiation in the range of 400-1,100 nm, while a liquid nitrogen-cooled Ge:Au photoresistor was used to register IR radiation (2-11 qm). After detection of laser radiation, an IEK-1 calorimeter was used to measure the energy of the laser pulse.

Initially, the active medium used was a He-Xe mixture (10:1) at pressures of 0.08-1 atm. An oscillogram of one of the first experiments at a mixture pressure of 1 atm is shown in Fig. 2.7. The laser wavelength, determined approximately using light filters was ~3 qm. The power of the laser radiation in the mode optimal for pressure and composition was 25 W with an efficiency of ql ~ 0.5 % with respect to the energy absorbed in the mixture.

In the next series of experiments on the pulse reactor TIBR-1 M (1974-1976), uranium layers more resistant to mechanical loads were used, and the laser cell was arranged horizontally, which made it possible to eliminate the uranium-dust con­tamination of the lower spherical mirror that had been observed in prior experiments.

The core of the TIBR-1 M reactor includes a ZrH19 moderator layer, which in comparison with other fast neutron reactors, leads to a reduction in dynamic loads on the fuel elements of the reactor core and to an increase in the duration of the reactor pulse to ~500 qs [2]. The diameter of the uranium-molybdenum alloy reactor core is around 30 cm.

The cylindrical laser cell, enclosed by a polyethylene moderator with a layer of 235U3O8 around 9 mg/cm2 thick deposited to the internal surface, was arranged close to the surface of the reactor core (see Fig. 2.8). In order to reduce the effect of thermal neutrons emerging from the moderator, on the reactor core the laser cell was surrounded by screens made of cadmium and boron carbide.

A diagram of the experiment is shown in Fig. 2.9. The measuring equipment (except for the IEK-1 calorimeter) was placed on the other side of the biological shielding in a neighboring room. To register the laser radiation, along with the IEK-1 calorimeter, Ge:Au photoresistors and DKPs surface-barrier silicon diodes were used [49]. The length of the uranium layer in the cell was 57 cm, with a diameter of 2.7 cm. The cell was irradiated with a pulsed flux of thermal neutrons with a half-height pulse duration of around 0.8 ms and an average fluence along the length of the uranium layer of 4.2 x 1013 cm~2, which made it possible at a helium pressure of 2 atm to obtain a specific power deposition at the pulse maximum of q = 600 W/cm3. In the experiments, binary mixtures of rare gases were studied: He-Ne (Ar, Kr, Xe), Ne-Ar(Kr, Xe), Ar-Kr(Xe), and Kr-Xe. The total pressure of the mixtures was equal to one atmosphere, and the partial ratio of components ranged from 200:1 to 200:30. Lasing was obtained with use of the mixtures He-Ar(Kr, Xe) and Ar-Kr(Xe) in a range of 2-10 ^m, and the mixtures He-Ar and Kr-Xe in a range of 0.2-1.2 ^m (see Fig. 2.10).

In this series of experiments [5, 50], the most intensive laser transitions were found for ArI (X = 1,15; 2.40 ^m), KrI (X = 2.52 ^m), XeI (X = 2.6 ^m), and thus the existence of the family of NPLs operating on IR transitions of atoms of rare gases was obtained. For the NPLs that were studied in greatest detail, based on mixtures He-Xe (X = 2.6 ^m) and He-Ar (X = 1.15 ^m), laser powers of 2,000 W and 250 W were found for Ці = 0.8 and 0.1 %, respectively.